The present invention relates to a solid-state imaging device and, more particularly, to a contact-type image sensor having a cell array of amorphous semiconductor photoconductive elements which are switch-driven by means of a matrix drive technique.
Amorphous semiconductor image sensors have been used in a wide variety of optical image-reading apparatuses, such as facsimiles, optical character readers (OCR), and copying machines, since their use makes it possible to minituarize these apparatuses. Considerable progress has been made in developing contact-type solid-state linear image sensors which have substantially the same length as the width of a paper document to be read. Such image sensors are advantageous over other imaging devices in that they can eliminate image reduction using a lens system for reducing the image of a document before the document image reaches the image sensors. Hence, they contribute to the miniaturization of optical image-reading apparatuses.
In the contact-type amorphous semiconductor image sensor, a cell array of amorphous semiconductor photoelectric detecting elements is connected to a matrix wiring circuit, and is switch-driven by means of a matrix drive technique. One such contact-type amorphous image sensor is disclosed in U.S. patent application Ser. No. 943,705 (Kouhei SUZUKI et al.), filed Dec. 19, 1986. This image sensor has linearly aligned, photoelectric detecting elements serving as picture elements (pixels). These elements are divided into a predetermined number of cell units connected to an image signal detector and a drive voltage-generating circuit. The image signal detector has a load resistor and an amplifier. The load resistor generates a voltage which changes as the image current signals sequentially supplied to the resistor change. The amplifier amplifies the voltage generated by the resistor, thus providing time-sequential image readout signals.
The contact-type amorphous semiconductor image sensor, which outputs the voltage generated by the load resistor as image signals, does, however have the drawback that it cannot produce time-sequential image signals as quickly as is required. This is inevitable, since the time constant .tau. of the image signal detector cannot be sufficiently reduced. The time constant of the image-signal detector is determined by the product of the resistance R of the load resistor and the capacitance of the electrostatic capacitor inevitably present and coupled in parallel to the load resistor. (Capacitance is parastic capacitance, stray capacitance and/or coupling capacitance.) Since the electrostatic capacitance cannot be infinitely reduced, then, in order to reduce time constant .tau., it is generally necessary to reduce resistance R of the load resistor. However, if resistance R is decreased, the voltage which can be generated in the load resistor will be reduced. Consequently, the effective component of each image signal output by the image sensor will decrease, thereby reducing the signal-to-noise ratio of the image signal. In the case of an amorphous silicon image sensor, in particular, the voltage which can be generated by the load resistor is significantly lowered. This is because the amorphous silicon forming the photoelectric converting film of this image sensor has a low electroconductivity (a low photosensitivity), though it is superior to that of cadmium sulfide-selenium which forms the photoelectric converting film of a matrix-driven image sensor. Hence, it is an extremely great problem that the signal-to-noise ratio of the image signal decreases when the resistance of the load resistor used in the contact-type amorphous semiconductor image sensor is lowered in order to produce time-sequential image signals at high speed.